1. Field of the Invention
The present invention generally relates to a method of controlling calls in a mobile communication system, and particularly relates to a method of controlling calls when a base station receives calls from a plurality of mobile stations. The present invention further relates to a base station which attends to communication based on such a method of controlling calls.
2. Description of the Related Art
In mobile communication systems, the number of simultaneously connectable users (hereinafter referred to as “capacity”) differs depending on employed wireless access schemes. In a mobile communication system using FDMA (frequency division multiple access) as a wireless access scheme, for example, capacity is determined by a frequency band occupied per user and a frequency band available to the system. If the wireless access scheme is TDMA (time division multiple access), capacity is determined by the number of timeslots that are the unit of division used for time division. If the wireless access scheme is code division multiple access such as CDMA, capacity is determined by electric power occupied by each user and the amount of interference between users. Factors that determine the capacity of the CDMA scheme include the number of codes that can be allocated, the amount of interference on the uplink, and the transmission power on the downlink. The amount of interference differs depending on the characteristics of employed codes. In any event, the capacity basically corresponds to a point at which the total of user electric power exceeds a certain limit.
Normally, a base station used in a mobile communication system accepts calls from mobile stations (i.e., from users) by taking into account the capacity that is determined as described above. The call admission operation performed by the base station is carried out by following a procedure as shown in FIG. 9, for example. In this description relating to FIG. 9, the base station is assumed to be employing the CDMA scheme.
In FIG. 9, upon receiving a call from a mobile station, the base station checks whether there are available codes that can be allocated (S2). If the check finds that there are available codes (YES at S2), a check is further made as to whether the amount of uplink interference stays below a predetermined amount (S4). If the check finds that the amount of uplink interference stays below the predetermined amount (YES at S4), a further check is made as to whether a sufficient transmission power is left in the base station, i.e., whether the transmission power of the base station stays below a threshold (S8). If the check finds that the transmission power of the base station stays below the threshold (YES at S8), a call setting process is performed for the call that has been received. If any one of the checks (S2, S4, S8) does not satisfy the required conditions, a call loss process is carried out (S9, S10, S11).
FIGS. 10A and 10B are charts showing temporal shifts in the base station transmission power.
FIG. 10A shows a temporal shift of base station transmission power in a case where calls based on communication through circuit switching are exclusively connected to the base station. FIG. 10B shows a temporal shift of the base station transmission power in a case in which calls of circuit switching and calls of packet switching are mixed and connected to the base station. In FIGS. 10A and 10B, a horizontal axis represents time, and a vertical axis represents a base station transmission power (%). In this example, the number of simultaneously connected calls is assumed to be 25, and the powers of all the connected users are added to the total. In FIG. 10A and FIG. 10B, an average and a variance of the base station transmission power computed over time 0 through time 250 are shown at the bottom right portion.
As shown in FIG. 10A, where calls of circuit switching are solely connected, the waveform of the base station transmission power does not exhibit extreme changes. Where calls of circuit switching and packet switching are mixed, as shown in FIG. 10B, the waveform of the base station transmission power exhibits volatile changes. This is because the transmission of packet switching calls is performed in a burst-like manner. That is, transmission is made when data is present, and stops when no data is present, such intermittent nature of transmission causing the volatile changes.
As described above, among various checks that have to be made in the base station in response to reception of a call, the number of available codes that can be allocated (i.e., the check at S2 of FIG. 9) is determined by the number of currently connected users. It is safe to assume that no mistake occurs in counting the number of available codes. The amount of uplink interference and the transmission power of a base station, however, may exhibit volatile changes depending on the conditions of connected calls as shown in FIGS. 10A and 10B. For example, even if a sufficient transmission power remains at the time of detection of the base station transmission power (at S5 of FIG. 9), it is possible to end up accepting an excessive number of calls unless calls are accepted by taking into account the types of currently connected calls (i.e., whether calls of circuit switching or calls of packet switching). It is possible that a large number of packet switching calls have been already connected but happen to be currently inactive when the base station accepts further calls. In such a case, the base station transmission power may become insufficient.
Accordingly, there is a need for a call admission control apparatus and a base station which accept calls without degrading communication quality by monitoring the transmission power levels properly.